Mozilla officially launched Firefox 4 Tuesday at around 10:00 a.m. EDT, bagging an even million in the first three hours. According to Mozilla's real-time calculator , about 6.5 million copies of Firefox 4 had been downloaded by 10:00 a.m. Wednesday.

But while Firefox 4's first-day count trounced IE9's, it fell short of the record established by Firefox 3.0 in mid-2008 when that browser was downloaded more than 8 million times. Then, Mozilla had pushed for a Guinness World Record with a "Download Day" campaign that urged users to flood its servers with requests. Firefox's Guinness record still stands.

Not surprisingly, Firefox 4 is doing best in Europe, long a Mozilla stronghold.

Of the 7.6 million downloads counted by 11:30 a.m. ET, 44 percent went to users in Europe, while North America accounted for 26 percent. Asia came in third, with 20%, with South America, Africa and Oceania all in single digits.

Artwork: Chip TaylorTuesday's release marked the end of more than a year of development. Mozilla issued the first alpha edition of the browser in February 2010; in July it kicked off a series of betas that eventually ran to an even dozen.

Firefox 4 was originally scheduled to ship last November, but bugs and other delays forced it to push the release into early this year.

It should come as a shock that Firefox 4 has trumped IE9 in the first day download contest: Firefox 4 has a built-in edge because it runs on Windows XP, the 10-year-old operating system that IE9 doesn't support.

Web metrics firm Net Applications said last month that XP accounts for more than 61% of all copies of Windows used on machines that went online in February.

Mozilla and Microsoft have recently traded barbs over the latter's decision to dump XP. Microsoft has defended the move by saying it did not want to develop for what it called "the lowest common denominator," a harsh label for the world's most successful operating system.

"The browser is only as good as the operating system it runs on and a browser running on a ten-year-old operating system tethers the Web to the past," a Microsoft spokesman said yesterday.

Earlier this week, Mozilla knocked its rival for leaving XP users stuck with the two-year-old IE8, and with no hope for a Microsoft-made browser upgrade.

Firefox could use the jumpstart provided by version 4: Net Applications' statistics show that the browser, once a lock to grab a quarter of the worldwide browser usage share, has stalled since its November 2009 peak. In the last 12 months, Firefox has lost two-and-a-half percentage points of share, and now accounts for 21.7% of all browsers in use.

During that same 12-month period, IE has lost 4.8 percentage points, while Google's Chrome gained 5.3 points and Apple's Safari grew by 1.9 points.

Users can download Firefox 4 for Windows, Mac OS X and Linux from Mozilla's site. People already running Firefox 3.5 or 3.6 can grab the upgrade by selecting "Check for updates" from the browser's Help menu.